Methods for the oxygen-based delignification of pulp

ABSTRACT

Pulp is delignified by forming a mixture of pulp and caustic, adding oxygen and feeding this oxygen-containing mixture to a first step reactor. The lignin in the oxygen-containing mixture will be partially delignified and will be fed to a second step reactor where the remainder of the lignin in the pulp will be delignified. In this manner, fast reacting lignin can be treated in the first step reactor and slow reacting lignin can be treated in the second reactor. The delignified pulp is recovered from the apparatus and after washing can be forwarded to a bleaching unit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. provisionalapplication 62/020,446 filed Jul. 3, 2014.

BACKGROUND OF THE INVENTION

Oxygen delignification is the step between digesting wood chips in pulpmaking and pulp bleaching operations. Oxygen delignification is designedto dislodge and disintegrate residual lignin left in the pulp after thedigestion step using oxygen and alkali. It is the direct extension todelignification that occurs during digestion. Contacting pulp in anaqueous alkaline medium with oxygen causes oxidation of lignin which notonly breaks molecules but also makes it water soluble. Oxidation ofcolor imparting groups reduces the Kappa Number lowering the consumptionof bleach chemicals in the bleach plant. Delignification with oxygen isa more gentle way of reducing the Kappa Number than by extendeddigesting and bleaching. In recent decades, new pulp mills have beenincreasingly adopting oxygen delignification systems as an advantageousstep in reducing environmental impact and achieve a better return oneconomic investment.

The most commonly practiced oxygen delignification consists of thefollowing steps. The first step involves adding washed pulp into amixer, adding caustic, adding oxygen and steam to bring the temperatureto a range of 70° to 95° C. and introducing this pulp mixture into thebottom of a vertical tall reactor in a continuous fashion. The pulp willflow upwards while lignin in the reactor gets oxidized in the alkalinemedium thereby dissolving and disintegrating the lignin and dislodgingit from the pulp fibers. The reactor is maintained at 5 to 10 barpressure to improve on oxygen update. The residence time for pulpflowing through a commercially practiced reactor is in the range ofbetween 20 and 100 minutes.

Oxygen delignification can be performed with both medium as well as highconsistency pulp. Due to limited effectiveness, difficulty in mixing ofoxygen and other operating problems with high consistency pulp, oxygendelignification has not achieved widespread success when compared withmedium consistency pulp.

Oxygen delignification works with pulps from both types of woods,hardwood and softwood, reducing the Kappa Number up to 35% and 50%respectively. In the case of hardwood, a two stage approach is neededwhere two reactors are placed in series. The first stage is maintainedat a higher pressure and lower temperature with less residence timewhile the second stage is usually maintained at lower pressure but athigher temperatures and greater residence times.

There has been limited success using a single stage high efficiencyreactor towards achieving short term delignification at smaller andexperimental scales. Consequently, there has not been any commercialapparati embodying this concept.

SUMMARY OF THE INVENTION

In a first embodiment of the invention, there is disclosed a method fordelignifying pulp comprising the steps of:

-   -   (a) Forming a mixture of the pulp and caustic;    -   (b) Feeding oxygen to the mixture and feeding the        oxygen-containing mixture to a first step reactor wherein lignin        will delignify;    -   (c) Feeding the oxygen-containing mixture to a second step        reactor wherein lignin will delignify; and    -   (d) Recovering the delignified pulp.

In step b) the fast reacting lignin or that lignin which contains themore fast reacting components will delignify first when mixed with theoxygen. This delignified lignin as well as that lignin that was notdellgnified will be passed through with the remainder of the pulp to thesecond step reactor in step c) where the lignin containing the slowreacting components will delignify. Some fast reacting lignin may ofcourse carry over from the first step reactor to the second step reactorwhere it will delignify along with the slow reacting lignin.

The pulp that is treated is selected from the group consisting of mediumand high consistency pulp.

The caustic employed is typical of that used in pulp and paperoperations and typically is sodium hydroxide.

The oxygen is generally pure oxygen but oxygen concentrations greaterthan 80% purity will be effective for creating the oxygen-containingmixture. Supplemental oxygen may be added to the mixture of pulp andcaustic after step (b) and before the oxygen-containing mixture is fedto the second step reactor.

Steam may be added to the mixture of pulp and caustic before being fedinto the first step reactor in order to increase its temperature.

The first step reactor is a reactor where the gas phase is thecontinuous phase and the liquid phase is the dispersed phase.

The oxygen-containing mixture is present in the first step reactor forabout 0.25 to 5 minutes.

The second step reactor is preferably a reactor column.

The oxygen-containing mixture is present in the second step reactor forabout 2 to 40 minutes.

After the delignified pulp is recovered in step (d), it may be fed to awashing unit where water may be added to the washing unit.

The delignified pulp may also be fed to a pulp bleaching operation.

In another embodiment of the invention, there is disclosed an apparatusfor delignifying pulp comprising a mixer, a first step reactor and asecond step reactor.

In the apparatus the first step reactor is a reactor where the gas phaseis the continuous phase and the liquid phase is the dispersed phase.

The second step reactor is a reactor column.

The mixer is in fluid communication with the first step reactor whilethe first step reactor is in fluid communication with the second stepreactor.

This apparatus can further comprise a washing unit

Studies have demonstrated that the addition of oxygen utilized indelignification will also cause oxygen to react with cellulose andhemicelluloses and no-lignin components in the pulp system.

The residual in the pulp after digestion is often characterized ascontaining “fast reacting” and “slow reacting” lignin. Many non-ligninbased oxidizable structures present in the pulp that consumes oxygenhave been described by pulp and paper scientists.

The present invention divides the oxygen consuming reaction in oxygendelignification as “fast” and “slow” oxidation without specifying themto be either specific lignin or non-lignin reactions. Thus fast reactinglignin will be that part of the lignin in the pulp that contains more ofthe faster reacting components when the lignin is delignified. The slowreacting lignin will be that part of the lignin in the pulp thatcontains more of the slower reacting components when the lignin isdelignified.

The methods of the present invention achieve high levels of oxygendelignification by using two steps. In the first step, a reactor isemployed that offers high efficiency with respect to mass transfer. Thereactor is capable of transporting oxygen from the gas phase to theliquid phase at extremely high rates. The pulp that is introduced intothe first reactor has the highest concentration of both the fast and theslow reacting components. Oxygen is a sparingly soluble gas and byutilizing a high efficiency mass transfer reactor and elevated pressure,the aqueous medium surrounding the pulp can be maintained to saturationlevels which maximize and maintain rapid reaction rates between the fastreacting components and oxygen.

The residence time of the pulp in the first step reactor or the feedrate of the pulp to the high efficiency reactor is adjusted accordinglyto ensure depletion of almost all the fast reacting components or fastreacting lignins of the pulp before it exits the reactor.

The typical high efficiency reactor may offer 10 to 1000 times greatermass transport per unit volume compared to conventional gas-liquidreactors such as continuously stirred tank reactors or mechanicallyagitated contactors, bubble columns, etc.

The reactor employed for the first step of the invention is one wherethe gas phase is the continuous phase and the liquid phase is thedispersed phase. The power required for the gas liquid contacting isprovided to the gas phase. In such a reactor, the gas phase hasextremely high turbulence for transporting gas to the aqueous phase andonly a partial amount of the energy from this turbulence is conveyed tothe liquid phase thereby avoiding undesirably loss of fiber strength ofthe pulp.

Some of the manufacturers of such high efficiency reactor's attributeunusually high mass transport to ultrasonic phenomena. One such reactoravailable is made by Quantum Technologies of Akron, Ohio. However, forpurposes of the invention, the method described herein is not limitedsolely to this one reactor.

Once the fast reacting components are oxidized, the fast mass transferof oxygen of gas to the aqueous phase is not critical. The slow reactingcomponents can be oxidized by holding the pulp exiting the first step ina chamber or a column type reactor with a predefined residence time asthe second step of the invention. As discussed previously, the pulpleaving the first step will always be saturated with oxygen if operatedas described for the first step. Since in the second step, the oxidationreaction is very slow, supplemental oxygen is limited and also veryslowly consumed. Providing supplemental oxygen in the second step can beaccomplished by allowing the complete output from the first stepcontaining both phases. The gas phase consisting of excess oxygen andthe aqueous phase consisting of pulp can be introduced at the bottom ofthe chamber or column utilized in the second step. Both the aqueous andgas phase in the chamber or column of the second step will flowco-currently upward. As the gas phase is lighter, it flows upward in theform of bubbles faster and rises through the aqueous pulp in thechamber/column supplementing the depleted oxygen and agitating the pulp.

The pulp exiting the second step reactor is then washed and sent tofurther processing steps such as bleaching.

The combination of the first step and the second step provide highefficiency oxygen delignification at lower costs than conventionaltechniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The figure is a schematic of a pulp delignification process according tothe methods of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the figure, a schematic of a pulp delignification systemaccording to the methods of the invention is shown. A mixer A receivespulp through line 1 and caustic through line 2. The combination of pulpand caustic is mixed in the mixer A and fed through line 3 and a pump(not shown) to the first step reactor B. The pump will raise thepressure of the pulp and caustic mixture.

Oxygen is fed through line 4 to join with the pulp caustic mixture inline 3 before being fed into the first step reactor B. Steam may also beintroduced upstream of the first step reactor B to raise the pulp andcaustic mixture temperature to between 50° C. and 100° C.

As discussed, there is an improved rapid reaction between the fastreacting lignin components or fast reacting lignin and the oxygenthereby resulting in the depletion of all the fast reacting componentsof the pulp before it exits the first step reactor B.

The pulp which now contains mostly the slow reacting lignin componentswill be fed from the first step reactor B through line 5 into the bottomsection of the second step reactor chamber or column C. The second stepreactor chamber or column C is designed to allow for additionalretention time for the pulp to contact the oxygen thereby delignifyingthe slow reacting components.

The excess oxygen from the first step reactor B will be adjusted so asto provide the required oxygen feed rate in the second step reactorchamber or column C.

The gas and aqueous phase streams will rise through the reactor chamberor column C and the gas will be exhausted from the top through line 10.The mix of water and delignified pulp will be fed from the second stepreactor chamber or column C through line 6 and will be washed in washerD which is assisted in this by the addition of fresh water through line7. The pulp will be separated from the water and fed through line 9 to apulp bleaching operation (not shown). The remaining water from thewashing step can be discharged from the washing unit D through line 8where it can be reused or cleaned up prior to discharge into theenvironment.

The advantages of the present invention are manifest in a number ofareas. A smaller reactor column can be employed thereby reducing thecapital required for equipment. Due to residency time of between 0.25and 5 minutes for the first step reactor and 2 to 40 minutes for thesecond step reactor, the amount of feedstock in progress in the vesselis lessened.

The equipment is one twelfth to one half the size of conventional oxygendelignification equipment making for an easier retrofit into existingmills that do not have oxygen delignification.

The methods of the present invention will provide for a better KappaNumber reduction that that achieved by traditional oxygendelignification systems.

The pulp produced by this invention has better pulp strength as itexposes the cellulose fiber in the pulp to the caustic and oxygen forshorter durations of time.

In depth analysis of feed stock and its complex chemistry is notnecessary to implement the two step method of the invention.

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of the invention will be obvious to those skilled in theart. The appended claims in this invention generally should be construedto cover all such obvious forms and modifications which are within thetrue spirit and scope of the invention.

Having thus described the invention, what I claim is:
 1. A method fordelignifying pulp comprising the steps of: (a) Forming a mixture of thepulp and caustic; (b) Feeding oxygen to the mixture and feeding theoxygen-containing mixture to a first step reactor wherein lignin willdelignify; (c) Feeding the oxygen-containing mixture to a second stepreactor wherein lignin will delignify; and (d) Recovering thedelignified pulp.
 2. The method as claimed in claim 1 wherein the ligninin step b) is fast reacting lignin.
 3. The method as claimed in claim 1wherein the lignin in step c) is slow reacting lignin.
 4. The method asclaimed in claim 1 wherein the pulp is selected from the groupconsisting of medium and high consistency pulp.
 5. The method as claimedin claim wherein the caustic is sodium hydroxide
 6. The method asclaimed in claim 1 wherein the oxygen is greater than 80% purity.
 7. Themethod as claimed in claim 1 wherein steam is added to the mixture ofpulp and caustic before being fed into the first step reactor.
 8. Themethod as claimed in claim 1 wherein the oxygen-containing mixturecomprises a gas phase and a liquid phase.
 9. The method as claimed inclaim 8 wherein the first step reactor is a reactor where the gas phaseis the continuous phase and the liquid phase is the dispersed phase. 10.The method as claimed in claim 1 wherein supplemental oxygen may beadded to the mixture of pulp and caustic after step (b).
 11. The methodas claimed in claim 1 wherein the oxygen-containing mixture is presentin the first step reactor for about 0.25 to 5 minutes.
 12. The method asclaimed in claim 1 wherein the second step reactor is a reactor column.13. The method as claimed in claim 1 wherein the oxygen-containingmixture is present in the second step reactor for about 2 to 40 minutes.14. The method as claimed in claim 1 wherein the delignified pulp is fedto a washing unit.
 15. The method as claimed in claim 13 wherein wateris added to the washing unit.
 16. The method as claimed in 13 whereinthe delignified pulp is fed to a pulp bleaching operation.
 17. Anapparatus for delignifying pulp comprising a mixer, a first step reactorand a second step reactor.
 18. The apparatus as claimed in claim 17wherein the first step reactor is a reactor where the gas phase is thecontinuous phase and the liquid phase is the dispersed phase.
 19. Theapparatus as claimed in claim 17 wherein the second step reactor is areactor column.
 20. The apparatus as claimed in claim 17 wherein themixer is in fluid communication with the first step reactor.
 21. Theapparatus as claimed in claim 17 wherein the first step reactor is influid communication with the second step reactor.
 22. The apparatus asclaimed in claim 17 further comprising a washing unit.